Re: [PATCH v3] s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 15.12.20 11:57, Halil Pasic wrote:
> On Mon, 14 Dec 2020 11:56:17 -0500
> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
> 
>> The vfio_ap device driver registers a group notifier with VFIO when the
>> file descriptor for a VFIO mediated device for a KVM guest is opened to
>> receive notification that the KVM pointer is set (VFIO_GROUP_NOTIFY_SET_KVM
>> event). When the KVM pointer is set, the vfio_ap driver takes the
>> following actions:
>> 1. Stashes the KVM pointer in the vfio_ap_mdev struct that holds the state
>>    of the mediated device.
>> 2. Calls the kvm_get_kvm() function to increment its reference counter.
>> 3. Sets the function pointer to the function that handles interception of
>>    the instruction that enables/disables interrupt processing.
>> 4. Sets the masks in the KVM guest's CRYCB to pass AP resources through to
>>    the guest.
>>
>> In order to avoid memory leaks, when the notifier is called to receive
>> notification that the KVM pointer has been set to NULL, the vfio_ap device
>> driver should reverse the actions taken when the KVM pointer was set.
>>
>> Fixes: 258287c994de ("s390: vfio-ap: implement mediated device open callback")
>> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
>> ---
>>  drivers/s390/crypto/vfio_ap_ops.c | 29 ++++++++++++++++++++---------
>>  1 file changed, 20 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
>> index e0bde8518745..cd22e85588e1 100644
>> --- a/drivers/s390/crypto/vfio_ap_ops.c
>> +++ b/drivers/s390/crypto/vfio_ap_ops.c
>> @@ -1037,8 +1037,6 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
>>  {
>>  	struct ap_matrix_mdev *m;
>>
>> -	mutex_lock(&matrix_dev->lock);
>> -
>>  	list_for_each_entry(m, &matrix_dev->mdev_list, node) {
>>  		if ((m != matrix_mdev) && (m->kvm == kvm)) {
>>  			mutex_unlock(&matrix_dev->lock);
>> @@ -1049,7 +1047,6 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
>>  	matrix_mdev->kvm = kvm;
>>  	kvm_get_kvm(kvm);
>>  	kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
>> -	mutex_unlock(&matrix_dev->lock);
>>
>>  	return 0;
>>  }
>> @@ -1083,35 +1080,49 @@ static int vfio_ap_mdev_iommu_notifier(struct notifier_block *nb,
>>  	return NOTIFY_DONE;
>>  }
>>
>> +static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
>> +{
>> +	kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
>> +	matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
> 
> 
> This patch LGTM. The only concern I have with it is whether a
> different cpu is guaranteed to observe the above assignment as
> an atomic operation. I think we didn't finish this discussion
> at v1, or did we?

You mean just this assigment:
>> +	matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
should either have the old or the new value, but not halve zero halve old?

Normally this should be ok (and I would consider this a compiler bug if
this is split into 2 32 bit zeroes) But if you really want to be sure then we
can use WRITE_ONCE.
I think we take this via the s390 tree? I can add the WRITE_ONCE when applying?